Batteries are energy storage devices that are well-known for use as an autonomous supply of energy for a desired application through a chemical reaction. Batteries are an energy dense technology (kWh/kg). Ultra-capacitors are increasingly used for supplying energy. Ultra-capacitors are designed to be very power dense (kW/kg) and are capable of delivering very high instantaneous current. Ultra-capacitors have a very simple construction when compared to batteries which leads to lower cost per unit of energy. Ultra-capacitors have an order of magnitude reduced internal resistance compared to batteries.
Hybrid energy storage systems using, for example, batteries and ultra-capacitors have been developed in order to take advantage of the strengths of each technology to provide high power density solutions. Such hybrids can be better optimized when compared to a single energy storage technology, like adding batteries in parallel to achieve peak power points of particular applications. The outcome of the combined design is that the total weight, volume and cost can be reduced over battery-only designs. In such previous hybrid energy storage systems, voltage control between the battery and ultra-capacitor sub-systems has been by active circuits (see FIG. 9A). Such previous designs commonly use a bi-directional DC/DC converter or an active current management control strategy.
The active controllers used in such hybrid energy storage systems add to the weight and complexity of the overall system. Additionally, the controllers are susceptible to electrical design risks common in space/aerospace applications.